1. Field of the Invention
The present invention relates to an asymmetrically coupled block copolymer and to a single batch process for its manufacture. The copolymer is useful in the production of tire components.
2. Description of Background
It is generally known that the properties of a rubber are determined to a far-reaching extent by the pattern of the tan delta curve. It is also known that a wide peak in the tan delta curve has a favorable affect on a series of antagonistic rubber properties such as improving the skid resistance, reducing the dynamic heat formation and lowering the rolling resistance without reducing abrasion resistance and providing continuous use on a daily basis.
In the past numerous attempts have been made to develop rubbers which have a high level of qualities. In order to attain as favorable as possible a compromise between the properties which, in part, are contradictory, mixtures of different rubbers have been employed. Another avenue which has also been followed is the use of copolymers prepared from two different polymer blocks. Thus, copolymers are known which differ in the nature of their chemical composition and/or their structure. Examples are styrene-butadiene block copolymers, the blocks A and B of which have different styrene and/or vinyl contents. The processing of such copolymers is relatively difficult. Another possibility of modifying the properties of rubber and in particular of improving the cold flow of rubber resides in coupling the so-called "living polymers" present towards the end of the polymerization with polyfunctional coupling agents such as polyhalogenides or polyalkenyl aromatic compounds. Often, however, the coupling yield, i.e., the percentage of coupled products in relation to the total amount of polymerization products is very low. According to this principle one invariably obtains coupling products, the arms of which are identical with respect to molecular weight composition and structure. In principle, coupling products are also known which have different arms. However, those products invariably contain blocks composed of styrene or butadiene units. Apart from the aforegoing, thermoplastic coupling products with different arms are known. To date there has been no report of use of such products in manufacture. For example, U.S. Pat. No. 4,248,983 discloses thermoplastic star-shaped block copolymers having the formula: EQU (A--B/A').sub.m --X--(A'/B).sub.n
which contains 60 to 95% styrene and 40 to 5% of a conjugated diene. Component A of the formula represents a nonelastomeric polymer segment which contains 80 to 90% styrene. A'/B and B/A' represent elastomeric polymer segments based on styrene and a conjugated diene. These products are manufactured by polymerizing the nonelastomeric segment A, adding an initiator, continuing the reaction by polymerizing a mixture of styrene and a diene and finally carrying out coupling. Similar block copolymers are known from U.S Pat. Nos. 4,180,530; 4,221,884 and 4,248,980 to 4,248,984.
U.S. Pat. No. 4,391,949 discloses coating compositions based on star-shaped block polymers of the formula: EQU (A--B).sub.x --Y--(C).sub.z.
Y in this formula represents a coupling agent, A the polymer of a monovinyl aromatic compound such as, for example, a polystyrene block, and B and C polymers of conjugated dienes, for example, polybutadiene blocks. The two polymers A-B and C are initially produced in two separate reactors. Thereafter, the contents of both reactors are combined and the coupling agent is added.
All known block copolymers are subject to at least one of the following shortcomings:
(i) The block copolymers do not adequately satisfy the increased demands placed upon them in their use as a tire material.
(ii) Problems arise regarding the compatibility of the two blocks.
(iii) The tan delta curve exhibits only a narrow damping maximum.
A need therefore continues to exist for rubbers which exhibit improved properties, particularly in their use as a tire material.